To minimize the amount of emissions exhausted into the atmosphere, modern automotive vehicles generally include one or more catalytic converters, or emission control devices, in the exhaust system of the vehicle. These catalysts store oxygen and NOx (collectively, “oxidants”) from the vehicle exhaust stream when the engine is operated with a relatively lean air/fuel ratio. On the other hand, when the engine is operated with a relatively rich air/fuel ratio, the catalysts release the stored oxygen and NOx, which then react with the HC and CO produced by the engine. In this way, the emission of both NOx and hydrocarbons (HC and CO) into the atmosphere is minimized.
The inventors herein have recognized that conventional systems suffer from NOx or hydrocarbon “breakthrough” of the catalytic converter. NOx breakthrough occurs when the catalytic converter stores oxygen and NOx for a period of time until the amount of stored oxygen and NOx exceeds the storage capacity of the catalytic converter. When this happens, any additional NOx produced by the engine will break through the catalyst and be emitted into the atmosphere, until a rich mixture is delivered to the catalyst and oxidation of the HC reduces the stored oxygen and NOx. Similarly, if the engine is operated with a rich air/fuel ratio for a period of time, and the oxygen and NOx stored in the catalyst is depleted, then any additional HC and CO produced by the engine will break through the catalyst without reacting with NOx or oxygen.
The inventors have further recognized disadvantages with the conventional approach to feedback air-fuel ratio control systems. In particular, the inventors have recognized that these systems attempt to maintain the engine at stoichiometry (or another desired air-fuel ratio). However, this has the disadvantage that engine air-fuel control is decoupled from the state of oxidant storage of the emission control device. The conventional system relies on air-fuel feedback to compensate for this oversight; however, this adds unnecessary delay and complexity.
The inventors have therefore developed a new method and system for adjusting the air/fuel ratio in the engine to minimize exhaust emissions by controlling the amount of oxidants stored in the catalyst around an oxidant target value instead of focusing on maintaining the air/fuel ratio around stoichiometry. The target value is the desired amount of oxidants stored in the catalyst. The inventors have further recognized that, in connection with this new engine control strategy, it is desirable to be able to control the oxidant storage capacity of the catalyst. However, even such a system can be improved.